A_guide_to_the_Jack2_source_code

Robin Gareus edited this page Feb 10, 2013 · 1 revision

This document should act as a link between the JACK design documentation and the source code. It is not supposed to be helpful if you do not look at the source code in parallel.

Shared memory

JACK2 uses the JACK1 source code for shared memory management. This code is encapsulated in an object oriented shared memory model in JACK2. The mother class for a shared memory management is JackShmMemAble. The classes JackClientControl and JackShmMem derive from it. JackShmMemAble is the base class for objects, which can be put in shared memory but do not have to be shared necessarily. JackClientControl is an example for such a class (the only one at the moment): Internal clients simply hold a normal instance of JackClientControl, while for external clients an instance of JackShmMem is created, as we see later in this chapter.

In JackShmMem the new and delete operators are overloaded, so that if you create a new object of a class inheriting from JackShmMem, not the normal dynamic object initialization routines are issued, but jack_shmalloc and jack_attach_shm (common/shm.c). It appeared that writing the fInfo field during the new operator failed with some compilers (the fields were erased at a later instant in the constructor's scope). For this reason, the overloaded new operator sets the info structure of the recently created shared memory segment to the global variable gInfo and gets it "written back" to fInfo during the function Init()} (common/JackShmMem.cpp}).

For client side use, there are the classes JackShmReadWritePtr, JackShmReadWritePtr1 and JackShmReadPtr, which are to be constructed with an index and the server name as parameters. They represent a shared memory object on client side. One can get the address of the shared memory segment the object represents by its function GetShmAddress() or by applying the dereferencing operator * on it. If one wants to access a member of the shared memory segments structure, he can also use the -> operator directly. One can even assign an integer to an object of one of these classes. This will set the integer as the new index of the object by calling the Init() function again.

JACK2 thread handling

The base class for thread representation in JACK2 is located in the namespace jack.detail and can be found in common/JackThread.h. The constructor of JackThread asks among others for the parameter JackRunnableInterface * runnable.

JackRunnableInterface is an abstract root class for objects which can be handed over to a new thread defining the actions to be be performed by the thread. It holds only two member functions: Init() and Execute(). While Init() is supposed to be called only once by the thread handler, the function Execute() should be called over and over again in an endless server loop. Such a thread handler, which should be set as the init point of the new thread (normally by Start()) can look like this:

void* JackPosixThread::ThreadHandler(void* arg)
{
    JackPosixThread* obj = (JackPosixThread*)arg;
    JackRunnableInterface* runnable = obj->fRunnable;
    ...
    obj->fStatus = kIniting;

    if (!runnable->Init()) {
        jack_error("Thread init fails: thread quits");
        return 0;
    }

    obj->fStatus = kRunning;

    bool res = true;
    while (obj->fStatus == kRunning && res) {
        res = runnable->Execute();
    }
    ...
}

In addition to Start(), Kill(), Stop() and AcquireRealTime(), a thread must also implement the static methods StartImp(pthread_t * thread, ...), StopImp(pthread_t thread), KillImp(pthread_t thread) to be called "out of the cold". The method StartSync() starts the thread, but in contrast to Start() it sleeps until the thread is started correctly.

The specific system depended thread implementations like JackWinThread or JackPosixThread inherit from JackThread. Please note that JackMachThread is not derived from JackThread directly but from JackPosixThread. Which one of these implementations is plugged into the interface is set up in the header $(platform)/JackPlatformPlug_os.h via typedef <implementation> JackThread. This is possible because the mother class JackThread resides in the subnamespace detail.

Lock-free shared memory access

To implement lock-free shared memory access JACK2 defines the class template

      template <class T> class JackAtomicState
        (common/JackAtomicState.h),

which offers lock-free access to an instance of the class T. For instance, the JackGraphManager is derived from JackAtomicState<JackConnectionManager>.

JackAtomicState has a two-element array of class T. One element holds the current state of the object, the other the next state. The next state can be written freely by non real-time threads, while still the real-time thread can read the status of the current state. If a non real-time thread wants to perform write access on the next state, it has to surround this quasi critical section with WriteNextStateStart() and WriteNextStateStop() commands (member functions of JackAtomicState). At the end of the cycle, the real-time thread tries to atomically switch the current and next state using CAS (Compare and Swap). It succeeds if no non real-time thread is in such a critical section, it fails otherwise. If it fails, one has to wait for the next cycle.

An important element of JackAtomicState is its member fCounter of the type AtomicCounter. The structure consists of a union called info. One representation of info is a UInt32 called fLongVal, the other is a structure of two UInt16, called fShortVal1 and fShortVal2

      =================================
      |            fLongVal           |
      =================================
      |   fShortVal1  |  fShortVal2   |
      =================================
      ^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^
           CurIndex        NextIndex
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                   Counter

      CurArrayIndex(e)  = CurIndex(e)       mod 2
      NextArrayIndex(e) = (CurIndex(e) + 1) mod 2

The header JackAtomicState.h introduces five macros

  • Counter(e),
  • CurIndex(e),
  • NextIndex(e),
  • CurArrayIndex(e),
  • NextArrayIndex(e) to make accessing AtomicCounter easier. The semantics of AtomicCounter can be described as follows: If fShortVal1 is equal to fShortVal2 (and therefore CurIndex == NextIndex), this means that either no update to the next state has happened yet or that a non real-time thread is currently writing to the next state.

The only thing TrySwitchState() does is atomically setting CurIndex(fCounter) to NextIndex(fCounter) using CAS principles. If CurIndex and NextIndex already were equal, nothing has changed. Otherwise CurIndex has a new value now (that one of NextIndex) and CurIndex is equal to NextIndex, indicating that no change to NextIndex has happened yet. WriteNextStateStart() atomically sets NextIndex(fCounter) to CurIndex(fCounter), causing TrySwitchState() to keep the current state. Additionally, it copies

      fState[CurArrayIndex(fCounter)]

to

      fState[NextArrayIndex(fCounter)] 

afterwards, if NextIndex(fCounter) and CurIndex(fCounter) already were equal before. WriteNextStateStop() increments NextIndex(fCounter) atomically, which indicates the next state has been changed but no thread is updating the next state at the moment so far.

API

The headers for the client API can be found in common/jack. They are taken from the C implementation and have remained almost unmodified. The JACK2 API implementation is located in common/JackAPI.cpp. JACK2 is designated to be compiled with the -fvisibility=hidden GCC option and equips all the API functions with the C++ visibility attribute

      (__attribute__((visibility("default"))))

But the export should only be done in the library itself, because the clients might not want to export the JACK1 API functions. Therefore, common/JackAPI.cpp does not include the API headers, but declares the API again with the visibility attribute attached.

Some of these API calls jack_client_open, jack_client_open_aux, jack_client_close, jack_get_client_pid) are different for external and internal clients. These functions are implemented in common/JackLibAPI.cpp and common/JackServerAPI.cpp, respectively. One might wonder why an internal client wants to call jack_client_open. An example for such a situation is a connected internal client, which wants to create another internal client.

Client representation

The client model of JACK2 contains four important classes:

  • JackDriverClientInterface,
  • JackExternalClient,
  • JackInternalClient and
  • JackLibClient.

All client classes are based on the JackClientInterface (common/JackClientInterface.h). Each external client is represented by a JackLibClient object on client side and a corresponding JackExternalClient in the server address space. Internal clients only need one representation, JackInternalClient, as the server address space is the context they run in. The JackDriverClientInterface will be covered in the drivers section.

JackClient---also derived from JackClientInterface---is an intermediate base class of JackLibClient and JackInternalClient. It encapsulates all the code shared between them. In addition, JackClient inherits from JackRunnableInterface. Its Execute() function is the clients real-time Process()} loop.

The client data, which is shared between the server and external clients, is put in a variable of the type struct JackClientControl. The shared memory segment is initialized during the Open()} function of JackExternalClient: First, the new operator of JackShmMem is called with the size of the struct JackClientControl as parameter. The resulting object is casted to its base class JackShmMemAble; the pointer to it is saved temporarily in an variable of the type JackShmMemAble *, named shared_mem. With the placement new operator, a new JackClientControl object is created. But instead of allocating new memory on the heap, this operator

      new(shared_mem)

stores the new object in the place given by the shared_mem variable. The pointer to the new object is stored in the fClientControl member of JackExternalClient.

To access the shared data, JackLibClient also holds an attribute called fClientControl, but with the type

      JackShmReadWritePtr1<JackClientControl>

The Open() function of JackLibClient initializes this value. The class JackInternalClient holds fClientControl as a simple member variable of the type JackClientControl.

Drivers

For the C++ reimplementation, a new driver interface was designed: This class JackDriverInterface can be found in common/JackDriver.h. Its important methods are Open(), Attach(), Detach(), Read(), Write(), Start(), Stop() and Process().

Together with JackClientInterface JackDriverInterface builds the base for the class JackDriverClientInterface (common/JackDriver.h) all drivers are derived from. JackDriverClientInterface does not add any members, but it is only used to join the two interfaces to a single one.

The function Read() / Write() is supposed to transport the data from / to the sound card to / from the driver clients output / input ports. The method Attach() / Detach() (de)allocates the capture and playback ports for the driver. Start() and Stop() should be self-explanatory. Process() is the function to call Read() and Write() and to activate the engine. It should be set as the real drivers callback routine (or should be called in the function set as a callback). Since some of these functions are the same for most of the drivers, this code is put in the class JackAudioDriver a driver should inherit from.

JACK works with callback based sound APIs like CoreAudio, which run a thread on their own, as well as with non-threaded APIs like ALSA. The common thread code for non-threaded mode driver APIs is located in JackThreadedDriver. This class is derived from JackDriverClientInterface and JackRunnableInterface and is constructed with the "real" non-threaded driver object as its only argument. JackThreadedDriver inherits from JackRunnableInterface, because it is also handed over to its member fThread as the task to be accomplished. On the driver part of JackThreadedDriver, most methods are directly redirected to the same method of the non-threaded driver (named fDriver); except for Start(), which first calls fDriver->Start() and then starts the drivers thread. The Init() function of JackThreadedDriver sets up the thread and driver parameters, the member Execute() is even simpler:

      bool JackThreadedDriver::Execute()
            { return (Process() == 0); }

IPC

In the JACK2 architecture, the IPC implementation is distributed amongst four different classes:

  • JackServerChannel,
  • JackClientChannel,
  • JackServerNotifyChannel and
  • JackNotifyChannel.

All of these classes are system dependent. The specific implementation is plugged in with the $(platform)/JackPlatformPlug_os.h header. The semantics of these classes are exemplified with the linux implementation using sockets.

The only instance of a JackServerChannel, fChannel, is held by the JackServer class. JackServerChannel implements the request channel endpoints on server side. The class JackSocketServerChannel both holds the entry point request socket fRequestListenSocket as well as a map from all open file descriptors to their socket representations. A JackServerChannel class is expected to start its own thread during Open(). Therefore, it makes sense for a JackServerChannel to inherit from JackRunnableInterface.

In contrast to JackServerChannel, a JackClientChannel implementation both has to take care of the notifications, which come from the server, and provide an interface for sending requests to the server. The expected JackClientChannel interface is defined explicitly in JackClientChannelInterface (common/JackChannel.h). These functions are called indirectly by the user via the client API (see the API section).

If a non real-time thread of the server wants to send a notification to clients, it will either use the JackEngine::NotifyClient() or JackEngine::NotifyClients(). The difference: The former informs only one single client, the latter iterates over all available clients. Both will call client->ClientNotify(). If the client has the type JackInternalClient, the function is called directly. If the client has the type JackExternalClient, its ClientNotify() function forwards the call to its member fChannel with the type JackNotifyChannel, which in turn has to forward the call via IPC to its corresponding JackLibClient. When the JackClientChannel of the JackLibClient receives the event, it will finally call ClientNotify() of the JackLibClient.

If the real-time thread of the server, the driver, wants to send a notification to clients, it has to use the function Notify()} of the member fChannel in the engine with the type JackSeverNotifyChannel. This function may fail and must be non-blocking, as it is used by the real-time thread. The implementation for Linux, JackSocketServerNotifyChannel, sends the notification as a request to the non real-time server thread, which delivers the notification to the clients / the client.